Live attenuated SIV vaccines which consist of molecular clones varying in tropism have been shown to protect against intravenous challenge with a primary isolate of SIV that consists of a genetic quasispecies. In our hands, these vaccines appear to induce sterilizing immunity because no evidence of the challenge virus can be identified by co-culture or PCR. To assess the role of antibody in the protection observed with these vaccines, we have performed passive protection experiments with IgG purified from pools of sera from monkeys infected with 2 congenic attenuated clones that vary in tropism Ron Desrosiers' SIVmac239 nef, which is T-cell tropic, and SIV/17E-Cl, which is macrophage-tropic. In addition, IgG preparations from a long-term survivor of SIV/DeltaB670 infection, along with a similar preparation from monkeys with progressive disease were also evaluated. The study consisted of 4 groups with 3 monkeys/groups. The SIV-specific antibody titer was determined in each sample by a Con A capture ELISA method by Montelaro's laboratory at the University of Pittsburgh School of Medicine and assayed for neutralization by Dr. Clements' laboratory at the Johns Hopkins School of Medicine against two stocks of SIV/DeltaB670 the challenge stock (B670Tulane) and a cell line passaged stock (B670Baltimore). Although B670Tulane is the challenge stock, it is easily neutralized. Neutralization of B670Baltimore, however, was correlated with protection in our earlier study and is therefore relevant to protection. Antibody avidity, as measured by the resistence of bound antibody to urea wash, was also determined by Dr. Montelaro's lab. Fifty-seven cc/kg IgG of SIV/17E-Cl and 50 ml/kg of SIVmac239deltanef antisera were given to each animal. Sixty ml/kg of SIV/DeltaB670 progressor and nonprogressor antisera was given to additional animals. The following morning, monkeys were challenged intravenously with 50 minimum I.V. doses of SIV/DeltaB670. By 14 days post-challenge, all monkeys had detectable virus in the peripheral blood. Four of 4 naive monkeys were also infected, validating the infectivity of the stock. In all the monkeys receiving SIV-specific antisera, both gp120 titers and antibody avidity were equivalent to that found in monkeys infected with attenuated virus and protected against i.v. challenge. Neutralizing titers to the challenge virus were also sufficiently high to afford protection. However, little, if any, neutralizing titer to the B670 Baltimore stock persisted in the recipient monkeys circulation at challenge. This phenomenon is not readily explained, but may reveal the failure of the purified IgG to afford protection. It is clear from previous studies that a serum component, identified by the ability of the serum to neutralize the SIV/DeltaB670 Baltimore stock, can prevent infection in naive recipients, even when given at 5 to 6 fold lower amounts. Purified IgG from this serum, however, does not prevent infection.. The genetic complexity of the virus appearing in the monkeys postchallenge was evaluated by heteroduplex mobility shift assays of PBMC DNA obtained during the first 4 weeks postchallenge. Interestingly, 9 of the 12 animals appeared to be infected with a single genotype. This finding was unexpected given the failure of the sera to prevent infection and the complexity of the virus stock used for challenge. The specific genotype found in these animals is currently being evaluated. The plasma virus loads in the recipients was also measured postchallenge by RT-QC-PCR. A significant reduction in virus load was noted at 7 days postchallenge in the 239delta nef and B670 nonprogressor groups; however, by 14 days p.i., all of the animals had virus loads equivalent to the naive controls. The effects of this parameter on immunosuppression is being determined by flow cytometric evaluation of lymphocyte subsets in the peripheral blood. Changes in these subsets, if they occur, will reveal disease progression; these should be apparent over the next several months.